This invention relates to radio broadcasting, and more particularly, to modulation formats for FM In-Band-On-Channel (IBOC) Digital Audio Broadcasting (DAB) and broadcasting systems utilizing such modulation formats.
Digital Audio Broadcasting is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. FM IBOC DAB can be transmitted in a hybrid format where the digitally modulated signal coexists with the currently broadcast analog FM signal. IBOC requires no new spectral allocations because each DAB signal is simultaneously transmitted within the spectral mask of an existing FM channel allocation. IBOC promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners. FM IBOC broadcasting systems have been the subject of several United States patents including U.S. Pat. Nos. 5,465,39; 5,315,583; 5,278,844 and 5,278,826. In addition, U.S. Pat. No. 5,956,624 for a xe2x80x9cMethod and System for Simultaneously Broadcasting and Receiving Digital and Analog Signals, by D. Kumar and B. Hunsinger, discloses an FM IBOC DAB system.
The advantages of digital transmission for audio include better signal quality with less noise and wider dynamic range than with existing FM radio channels. Initially the hybrid format would be adopted allowing the existing receivers to continue to receive the analog FM signal while allowing new IBOC receivers to decode the digital signal. Some time in the future, when IBOC DAB receivers are abundant, broadcasters may elect to transmit an all-digital format. The goal of FM hybrid IBOC DAB is to provide virtual-CD-quality stereo digital audio (plus data) while simultaneously transmitting the existing FM signal. The goal of FM all-digital IBOC DAB is to provide virtual-CD-quality stereo audio along with a data channel with capacity of up to about 200 kbps, depending upon a particular station""s interference environment.
Since there is expected to be a transition from hybrid to all-digital IBOC DAB formats, it would be desirable to devise a modulation format that can be used by both systems so that the transition can be made with minimal changes in transmitting equipment.
This invention provides a method of broadcasting comprising the steps of: providing a broadcast signal in a central frequency band of an FM radio channel; providing a plurality of upper sideband sub-carriers in an upper sideband of the FM radio channel, with the upper sideband lying within a frequency band extending from about +100 kHz to about +200 kHz from the center frequency of the radio channel; providing a plurality of lower sideband sub-carriers in a lower sideband of the FM radio channel, with the lower sideband lying within a frequency band extending from about xe2x88x92100 kHz to about xe2x88x92200 kHz of the center frequency of the radio channel; orthogonal frequency division modulating a first group of the plurality of upper sideband sub-carriers with a complementary punctured convolution coded version of a program to be transmitted; orthogonal frequency division multiplexing the plurality of lower sideband sub-carriers with the complementary punctured convolution coded version of the program signal to be transmitted; and transmitting the central frequency band signal, the plurality of upper sideband sub-carriers, and the plurality of lower sideband sub-carriers.
When operating with a hybrid modulation format, the central frequency band signal comprises a carrier FM modulated by an analog program signal. When operating with an all-digital modulation format, the central frequency band signal comprises a third plurality of sub-carriers modulated by a digital signal and broadcast at an average power spectral density less than that of the upper sideband and lower sideband sub-carriers.
In the all-digital format, this invention includes a method of broadcasting comprising the steps of: providing a first plurality of sub-carriers in an upper sideband of an FM radio channel; providing a second plurality of sub-carriers in a lower sideband of said FM radio channel; orthogonally frequency division modulating a first group of said first plurality of sub-carriers with a digitally coded version of program signal to be transmitted; orthogonally frequency division modulating a first group of said second plurality of sub-carriers with said digitally coded version of said program signal to be transmitted; providing a third plurality of sub-carriers in a central frequency band of said FM radio channel, wherein the power spectral density of the third plurality of sub-carriers is less than the power spectral density of the sub-carriers in the upper and lower""sidebands; and transmitting said first group of said first plurality of sub-carriers, said first group of said second plurality of sub-carriers, and said third plurality of sub-carriers.
The invention also encompasses transmitters and receivers that utilize the above methods.
This invention provides a modulation format that can be used by both hybrid and all-digital in-band-on-channel digital audio broadcasting systems so that the transition can be made with minimal changes in transmitting equipment.